This invention relates to synthetic quartz crystals, and, in particular, to quartz crystals having ultra high purity.
Quartz, chemically composed of silicondioxide and trace amounts of other materials such as aluminum and sodium and lithium alkalies, is one of the most abundant minerals in our geological environment. It often occurs in crystalline form and, most often, is colorless and translucent. Its crystallized structure is either right-handed or left-handed and rotates the plane of polarization of transmitted light. One of its properties, that make it especially useful for a wide variety of industrial and military applications, is its piezoelectricity. That is the crystal is capable of producing an electric charge on some of its surfaces when the crystal is compressed in certain directions. The charge disappears when the compression is removed. This particular property makes quartz crystals especially useful in microphones, phonograph pickups, ultrasonic generators and in electromechanical devices, such as frequency-controlling quartz crystal resonators. These quartz crystal resonators are widely used in military and commercial communication systems for carrier-frequency separation and for selecting a desired frequency signal while rejecting undesired frequencies.
Generally, the piezoelectric quartz crystals used for industrial and military applications are synthetic in nature and grown commercially using a number of conventional crystal growing techniques. One of the more successful processes employed universally for synthetic quartz growth is the so-called hydrothermal technique. It is similar in theory to the well known solution growth technique and involves growing quartz crystals under high temperatures and pressures. Nutrient, or feed material, is dissolved in a high temperature region of a vessel. The solution is transported by thermal gradients to a lower temperature region, becomes supersaturated and the material precipitates on a quartz crystal seed in single crystal form. The temperatures involved in the hydrothermal process are higher than in solution growth and pressures of 1000 atmospheres or more may be used.
In the hydrothermal process, quartz growth takes place usually in a high pressure autoclave. The interior diameter of the autoclave is between 25-35 cm and the height between 3-5 meters. The nutrient, usually natural quartz chips, also called lascas, is placed in the bottom of the autoclave and separated from the top by a baffle containing small holes. Production autoclaves often use from 100-150 kg of nutrient per run. The quartz crystal seeds are suspended on racks positioned in the upper region of the autoclave which is at a lower temperature than the bottom. Typically, there are over 100 seeds in the autoclave. Thermal insulation is placed around the outside of the autoclave. All vessels used commercially are fabricated from steel and these are often buried in the ground or enclosed in heavy metal sheets as a safety factor.
Three crucial growth parameters, pressure, temperature, and fill are interrelated in the hydrothermal process, pressure being a function of temperature and fill. The fill, i.e., the percentage of the vessel volume filled with solid and liquid prior to sealing, is generally about 80 percent. At growth temperatures, owing to thermal expansion of water, the vessel is almost entirely filled with liquid. Fill is the most important parameter determining the average autoclave pressure. A ten percent change in fill will increase the pressure more than two fold at 350.degree. C. The fill for quartz has been varied from 65 to 88 percent corresponding to pressures of 150 to 3000 atmospheres. Most present commercial growth processes use up to 1500 atmospheres pressure.
The effect of temperature on pressure is less significant than fill, but temperature must be carefully controlled to prevent the pressure from exceeding autoclave mechanical design limits. Catastrophic failures have occurred even at commercial establishments. For alpha-quartz the growth, the growth T.sub.g near the seeds, can be varied from 200.degree. to 573.degree. C. At 200.degree. the reaction is very slow but above 573.degree. C. undesirable beta quartz is formed. Typical growth temperatures vary from 340.degree. to 375.degree. C.
The temperature differential T, between the two sections of the autoclave, the top part containing the seeds and the bottom part containing the nutrient, is significant to growth rate and can be varied between 5.degree. and 100.degree. C. In general however, the differential depends on the growth process and desired grade. More uniform crystals are obtained with smaller temperature differentials. The proper temperature differential, profile, and schedule for constant growth rate is usually established empirically for each autoclave.
The solubility of quartz in water under hydrothermal conditions ranges from 0.1 to 0.5 weight percent. These values can be increased to the desirable level of 2-5 percent by the addition of a mineralizer solution to the nutrient solution. The mineralizer solution can be either sodium hydroxide or sodium carbonate in the range of 0.5 to 1 normal. The growth rate of quartz is slow compared to most single crystal techniques being in the range of 20 to 40 mil/day. Thus, to produce material of the desired size growth runs can extend from 25 to over a hundred days.
The impurity of crystal produced by this process are on the order of 1 to 3, parts per million atomic by atomic absorption. The purity had been improved by oriented seed growth as disclosed in U.S. Pat. No. 4,576,808 which is incorporated by reference.
The use of silica nutrient as a starting material has not been used in the past because the much higher solubility of silica caused too much silicate to be in solution at the lower temperature used for growth. This is transported into the growth zone and is deposited on the seed at temperatures below the range of formation of high quality quartz. All subsequent growth at normal growth temperatures is deposited on this and is subject to strain and twinning.
Impurities and macroscopic inclusions (particulates) can be introduced into the growing quartz crystal by impurity ions present in the crystal growth environment. These may be contributed from the impurities present in the nutrient (pieces of natural quartz or synthetic quartz grown in a prior run), the mineralizer (sodium hydroxide or sodium carbonate in water, with a small amount of lithium ion often added to promote growth), the walls of the autoclave (a high tensile stainless steel) or the seed racks, baffles, baskets, and other hardware in the autoclave.
Most synthetic quartz is grown using natural quartz (lascas) as nutrient. The purity of this nutrient is subject to the conditions under which it was formed in nature, and can vary widely. A degree of purification may be achieved by using previously grown synthetic crystals as nutrient. Nevertheless, these were grown from natural quartz, and are subject to some of the same impurities. Quartz has also been grown in a two step process using silica as the source material. This involves a recrystallization of the glass under hydrothermal conditions in one step, removal of the crystallized product from the liner in which it was processed, and subsequent growth using this material as nutrient. The advantage of this is that silica glass can be obtained in almost any desired purity. Glasses produced by chemical vapor deposition have essentially undetectable concentrations of all metallic impurities. There are several disadvantages however: (1) Recrystallization of the quartz involves an additional step and requires dedicating an autoclave to this purpose for up to two weeks; (2) Much of the recrystallized material must be mechanically removed from the liner, resulting in a mixture of powders and various size pieces; (3) A recrystallization run will at most provide material for one and half to two growth runs; and (4) The product of the recrystallization run is microcrystalline and porous. It is very subject to contamination during storage and handling.
Because of these problems and the need for higher purity quartz crystals than previously produced, the search for another process and apparatus resulted in the present invention.